1. Field of the Invention
The present invention relates to a method and apparatus for measuring the concentration of gases using characteristic absorption techniques and, more particularly, to a method and apparatus for using a semiconductor laser as a variable wavelength light source for performing quantitative spectroscopic analysis of an unknown mixture of gases containing one or more analytes.
2. Description of the Prior Art
Generally, two different types of light sources have been used to measure the concentration of important analytes, such as oxygen, in a gas sample using characteristic absorption techniques whereby the irradiating light is absorbed at characteristic frequencies of one or more analytes in proportion to the amount of the analytes in the gas sample. The first type of light source is a broadband light source which illuminates the gas sample and is "tuned" to the desired characteristic absorption frequency by passing the light output through a narrowband filter "tuned" to the characteristic absorption frequency of the analyte of interest. However, such broadband light sources are generally undesirable for measuring oxygen concentration at 760 nm because such a system does not yield sufficient intensity modulation to make feasible an absorption technique for the detection of oxygen. Nevertheless, it is desired to measure the absorption of oxygen in the "A" region because no other gases are known to have spectral signatures in this region, thus minimizing interference. Accordingly, systems for measuring oxygen concentration typically use the second type of light source, which uses a narrowband light source such as a single mode or multi-mode laser precisely tuned to emit light in a narrow wavelength range about the desired absorption wavelength. However, such lasers are temperature dependent and, as a result, the temperature of the laser must be very precisely controlled to maintain the light output in the desired wavelength range. Unfortunately, large heat sinks are typically required for this purpose, which adds greatly to the bulk and expense of the light source. Also, even with such large heat sinks, precise control of the temperature, and hence the frequency of the light emitted by the laser light source, remains difficult.
An example of a system using a narrowband "tuned" light source is described by Kroll et al. in an article entitled "Measurement of Gaseous Oxygen Using Diode Laser Spectroscopy," Applied Physics Letters, Vol. 51, No. 18, Nov. 2, 1987, pp. 1465-67. Kroll et al. therein describe methods for using semiconductor lasers for gas spectroscopy and specifically for detecting the absorption of individual rotational lines of the A band of gaseous atmospheric oxygen in the spectral region of 760-770 nm and outputting a signal that is proportional to the O.sub.2 partial pressure. Kroll et al. use a "single" frequency laser with a nominal linewidth which is approximately 1% of the O.sub.2 linewidth and electronically scan the mode wavelength through individual rotational lines of the band and then measure the change in the light absorption. The light source is a laser diode with a nominal operating wavelength in the range of 759-764 nm which is mounted on a temperature-controlled heat sink and is driven by a current supply that provides DC bias, an adjustable 0-0.6 mA peak-to-peak sinusoidal modulation at 5 kHz, and an adjustable 0-1.2 mA ramp at 5 Hz. The output of the laser diode is collimated by a microscope objective, passed through the sample to be measured, and focused on a photodiode detector. During operation, the diode heat-sink temperature is slowly raised from 25.degree. C. to 40.degree. C. to produce a thermal scan. The thermal scan produces a discontinuous wavelength scan of approximately 30 Angstroms that covers the wavelength range containing several oxygen absorption lines. A particular line is examined by stopping the thermal scan and stabilizing the heat-sink temperature at the present level. Kroll et al. also suggest that the third derivative of the intensity of the photodiode detector can be used to derive an error signal to lock the laser frequency to O.sub.2 absorption line centers. Unfortunately, maintaining the heat-sink temperature at a given level, and hence maintaining the output at a constant wavelength, is quite difficult, as is locking the laser frequency using feedback techniques.
In U.S. Pat. No. 4,730,112, Wong describes a system which also uses a narrowband light source (laser diode); however, Wong further requires a reference filter with a narrow rejection (or transmission) band centered at the wavelength of interest. In particular, Wong describes a conventional oxygen measurement system which uses a tunable diode laser, such as a distributed feedback diode laser, whose emission wavelength is adjacent to but spaced from the wavelength of a distinct absorption line. The laser diode's drive current is altered in a feedback arrangement so that the junction temperature of the laser diode is changed, thereby changing the wavelength of the emitted radiation and, in effect, scanning the emitted radiation through a range of wavelengths that includes the absorption line of interest. The reference filter is used to assure that the laser light source locks onto the correct spectral feature so that the laser diode can be "locked" onto the characteristic absorption line for oxygen. A very limited amount of tuning of the laser is possible by varying the temperature of the diode laser as proposed by Wong, and, in any case, a very elaborate temperature control system is required to control the temperature to an accuracy of less than 0.05.degree. C. in order to select a wavelength within the few tenths of an Angstrom accuracy needed for gas spectrometry.
Wong further describes in U.S. Pat. No. 5,047,639 a concentration detector which uses a feedback loop to lock a single mode diode laser onto the chemical absorption peak of the analyte of interest. The diode laser purportedly produces a spectral distribution having a single peak of width comparable to the linewidth of the spectral lines in the gas being detected and is controlled to be coincident with one of the spectral lines in the gas being detected. As in the Wong '112 patent, the wavelength of the diode laser is controlled by controlling the drive current to the diode laser so as to control the temperature of the diode laser. The feedback loop varies the drive current until the wavelength of the diode laser is centered on an absorption peak of the reference gas component. The feedback loop includes dithering circuitry that produces a first harmonic component signal that is used to lock the laser wavelength to the center wavelength of the absorption peak of the reference gas and produces a second harmonic component signal that indicates whether the laser wavelength lies within an absorption peak. At device turn on, a ramp signal tunes the wavelength until the second harmonic signal indicates that the laser wavelength lies within an absorption peak. However, as with the Wong '112 system, a complicated feedback arrangement is required to "lock" the laser output to the wavelength of the absorption peak of the reference gas.
McCaul et al. describe in U.S. Pat. Nos. 5,448,071 and 5,625,189 an on-airway gas spectroscopy device which, as in the Wong systems, includes a tunable laser diode. However, in the McCaul et al. system, the laser diode is driven by a periodic stepped laser diode drive current where each period of the stepped laser diode drive current has a plurality of constant current intervals. Certain of the constant current intervals are used to lock the laser radiation emitted from the laser diode onto a preselected absorption line, while others of the constant current intervals are used to subtract baseline absorption measurements from peak constant current interval absorption measurements. By detecting absorption during the constant laser diode current intervals, McCaul et al. purport to separate the functions of subtracting baseline noise, centering radiation frequencies on the absorption line, and measuring the absorbance at the peak frequency of the absorption line. A heated sample cell is provided to allow for the measurement of oxygen concentration in human breath, where the pressure and temperature of the gas in the sample cell are detected to account for pressure and temperature dependencies of the absorption measurement. The laser light is directed through the sample cell multiple times to provide a sufficient path length for measurable oxygen absorption in the A region.
A semiconductor laser such as a vertical cavity surface emitting laser (VCSEL) is a laser which recirculates light inside the optical cavity so that the emitted light is normal to the surface of the laser. A cross-section of a conventional VCSEL is illustrated in FIG. 1. Like an edge-emitting laser, a VCSEL requires an active lightemitting layer 10 (50-300 Angstroms thick) to be sandwiched between two mirrors 20 and 30, which are, in turn, sandwiched between a p-contact 40 and an n-contact 50 separated from the lower mirror 30 by an n-doped substrate 60. However, in the case 30 of VCSELs, the mirrors 20 and 30 are part of the epitaxial layer design, and the length of the active layer 10 is four orders of magnitude shorter (typically 0.01-0.02 .mu.m). The shorter active length requires the use of many more reflective mirrors (e.g., distributed Bragg reflectors or DBRs). Generally, the lower reflective mirrors or DBRs 30 are doped n-type (with a reflectance R&gt;99.9%), while the upper reflective mirrors 20 are doped p-type (with a reflectance of approximately 99.5%) and include ion implants 70 so as to create a p-n junction with current flowing vertically through the device. The resulting device outputs light in wavelengths from 650 nm to 1100 nm and higher (763 nm in the example of FIG. 1).
Unlike LEDs or edge-emitting lasers, VCSELs can internally compensate their drive currents continuously over a temperature range and thus may be particularly suitable as a light source in analyte sensors. As illustrated in FIG. 2, the output wavelength of the VCSEL of FIG. 1 can be varied as a smooth function of device temperature for continuous wavelength tuning, while conventional edge emitting laser diodes exhibit mode hopping behavior in that they do not change wavelength in a smooth fashion. The wavelength change in the VCSEL occurs because of small changes in index of refraction and physical length of the resonating structures within the VCSEL.
In U.S. Pat. No. 5,570,697, Walker et al. describe an on-airway oxygen sensor which uses a VCSEL as a light source. The VCSEL is continuously tuned to emit light having a frequency linewidth of less than 3 GHz at the resonance of oxygen (in the 760 nm region) or any other analyte of interest. The VCSEL is driven by a wavelength controller which may be a resistor, thermoelectric cooler, or some similar device which can cause the laser beam wavelength to vary through an absorption resonance of the analyte of interest by varying the temperature of the VCSEL, as in the Wong et al. systems for laser diodes. Unlike conventional edge-emitting semiconductor laser diodes emitting at wavelengths shorter than 1200 nm which exhibit discrete "hops" in wavelength as the temperature or applied current is varied, the VCSEL is continuously tunable. Walker et al. tune the VCSEL to the maximum absorption line of the analyte of interest by providing electronic control circuits which correlate maximum and minimum absorption of the laser beam with the tuning signals applied to the VCSEL which produced the maximum and minimum absorptions. The VCSEL is then tuned so that it spends most of the time at a wavelength corresponding to the maximum absorption line of the analyte of interest, a lesser time at a wavelength corresponding to the nominally zero species absorption, and a minimum time at intermediate wavelengths. Alternatively, the VCSEL may be tuned using the third harmonic signal from a lock-in amplifier as described by Wong et al. Thus, as with the Wong et al. systems, a complicated feedback arrangement is required to "lock" the laser output to the wavelength of the absorption peak of the reference gas.
Thus, the basic principles of laser diode absorption spectroscopy, particularly as it applies to the measurement of gaseous oxygen, are known in the art. Walker et al. in particular discuss the benefits of using a VCSEL structure for spectroscopy. However, such prior art devices remain relatively expensive and unreliable because of the need to carefully control the temperature of the laser diode or VCSEL in order to "tune" to a particular wavelength for absorption measurements. A more reliable, less expensive way to perform laser spectroscopy in general and oxygen concentration measurements in particular is desired. Moreover, it is desired that the resulting light source be physically smaller and that it consumes less power than prior art devices. The present invention has been designed to meet these needs in the art.